The present invention relates to the production of flat substrates with an extent of at least 2500 cm2 and having a silicon layer deposited by means of a PECVD process in a vacuum reactor. Thereby, substrates are manufactured whereupon, generically, the silicon layer becomes part of semiconductor devices as in manufacturing flat substrates for thin film transistor displays (TFT) or for liquid crystal displays (LCD), or for solar cells or for organic light-emitting displays (OLED).
In today's manufacturing of such flat large substrates with a silicon layer, amorphous silicon (a-Si) is the most commonly used material for such layer. These layers are commonly deposited by a PECVD process. Unfortunately, however, a-Si has relatively poor electronic properties and a-Si-based devices on such flat substrates tend to show important degradations under electrical stress.
It is known in the art that crystalline silicon (μc-Si as well as “polycrystalline silicon”) could be a good candidate for replacement of a-Si, as devices made of more crystalline material exhibit better performance such as higher electron mobility (higher field effect mobility), higher ON-current and tend to degenerate less over time (threshold voltage shift).
It is also known in the art that highly crystalline material can be produced by plasma-enhanced chemical vapor deposition processes (PECVD) using standard equipment and plasma-activating a mixture of silicon-containing gas, halogen-containing gas, hydrogen and a noble gas. We refer, as an example to P. Roca i Cabaroca et al. in “Journal of SID” Dec. 1, 2004, where methods are reported for growing on top of a silicon nitride layer (SiN) μc-Si material from a SiF4—H2—Ar gas mixture. The problem inherent to this methods is that of poor uniformity of thickness and of quality for both, the underlaying dielectric layer—SiN—as well as the addressed μc-Si layers. The degree of uniformity of thickness and of quality is governed by the chemical and electrical homogeneity in the near-the-substrate environment. Indeed, both films, dielectric film and μc-Si film, have been found to be thinner and of different chemical composition at a peripheral portion of the substrate compared with these characteristics in the more central portion of the substrate.
It is an object of the present invention to provide a method for manufacturing flat substrates with an extent of at least 2500 cm2, substantially centrally symmetrically and having a Si layer deposited by means of PECVD processing in a vacuum reactor, whereat uniformity of at least one of layer quality and of layer thickness along the substrate up to its periphery is improved. This is achieved by such method which comprises:                a) generating an RF plasma discharge in a reaction space between the electrodes;        b) depositing on at least a part of the inner surface of the reactor a dielectric precoat;        c) introducing one substrate into the reactor with a first surface towards said first electrode;        d) depositing on a second surface of the substrate a dielectric layer;        e) PECVD depositing on said dielectric layer said Si-layer as a μc-Si layer and        f) repeating steps b) to e) for each single substrate to be manufactured.        
For the dielectric layer as deposited in step d) the thickness non-uniformity mainly results from the higher etching rate at the peripheral portion of the substrate also during subsequent Si-layer deposition. Etching radicals originate especially from the plasma used to grow the μc-Si layer in step e). The increase in etching rate towards the periphery of the substrate is believed to be caused by a higher proportion of these etching radicals near the edge of the substrate compared to such proportion in a more central area of the substrate. This can be understood knowing the good chemical neutrality of aluminum alloys with respect to fluorine, which alloys are customarily used for semiconductor fabrication vacuum reactor's wall. Such neutrality—lack of consumption—leads to an increased proportion of etching radicals along the surface of aluminum alloy of the reactor's inner wall and thus to an increased etching rate at the periphery of the substrate. It must be pointed out that the footprint of the vacuum reactor is customary dimensioned as small as possible in view of the dimension of substrates to be manufacture therein. Therefore the spacing between reactor's metallic wall and the edge or periphery of the substrate to be manufactured is not tailored so large that the addressed effect of such wall could be neglected.
Indeed, a most commonly used aluminum alloy for PECVD reactors is an aluminum magnesium alloy, because it develops a protective fluorine-based outer layer and can therefore easily withstand a cleaning step. Thereby, customarily fluorine radicals, as from NF3 or SF6 in plasma are used during such plasma-enhanced cleaning without corrosion of the vacuum reactor's inner surface for standard PECVD silicon processes.
Due to the chemical neutrality of the inner surface of the PECVD reactor the available quantity of etching radicals is higher near the edges of the substrate and these radicals migrate, as by osmotic effect, onto the substrate's peripheral area. The smaller that the footprint of the reactor's inner space is dimensioned compared with the dimension of the substrate, the more pronounced is this effect. The higher quantity of available etching radicals, fluorine radicals, will affect the μc-Si layer growth at the peripheral area of the substrate as by affecting crystallization. Therefore, the local perturbation of the chemical balance between etching- and growth-contributing radicals leads to lowering the deposition rate as well as changing the Si-material composition between defined crystalline fractions, amorphous fractions and voids at the peripheral area of the substrates. Generically, such decrease in layer thickness and change of material composition towards the edge of the substrate leads to undesired deviations of characteristics of semiconductor devices produced along the substrate and towards the periphery of the substrate, relative to characteristics of such devices more centralized on the substrate.
By depositing on at least a part of the inner surface of the reactor a dielectric precoat, before introducing the substrate, and then performing layer deposition upon the substrate, which includes depositing the μc-Si layer as a silicon layer, the inner surface of the vacuum reactor, which is metallic, as was addressed mostly of an aluminum magnesium alloy, is shielded from the precursor gas as used especially during deposition of the μc-Si layer. It is believed that thereby the proportion of etching radicals along the peripheral area of the substrate is kept substantially equal to such proportion in a more centralized substrate area. With respect to spatial relative density distribution of etching radicals, one will encounter substantially no difference in the central area of the substrate and along its periphery.
Thus, the principal which is followed up by the present invention is to present to the plasma activated gas substantially equal conditions, whether seen at the edge or more centrally of the substrate.
Attention is drawn to the following prior art documents: U.S. Pat. No. 5,177,578, U.S. Pat. No. 5,970,383, U.S. Pat. No. 6,071,573, U.S. Pat. No. 5,981,899, U.S. Pat. No. 5,811,195, JP 2002 289 557, P. Roca i Cabaroca et al. in “Journal of SID” Dec. 1, 2004.
The U.S. Pat. No. 5,970,383 teaches to deal with the time development of vacuum reactor wall coverings during manufacturing of series or batches of silicon-coated substrates by resetting the wall characteristics after each batch. Thereby, a coating is applied on the wall of the vacuum reactor. The influence of reactor's wall upon the local uniformity of the single substrate covering is not addressed.
The U.S. Pat. No. 5,981,899 teaches a capacitively coupled RF plasma reactor of the type as may be used to practice the method according to the present invention.
The U.S. Pat. No. 5,811,195 details the use of aluminum magnesium alloys for the walls of a vacuum reactors for semi-conductor processing.
For industrial processes a required uniformity of material properties and of thickness along large substrates of at least 2500 cm2 is important, considered over the whole substrate area. E.g. in layer deposition for TFT-devices, as used for manufacturing substrates for TFT display panels, the final properties of each transistor device of the backplane depend substantially from the thickness of the dielectric layer as deposited in step d) addressed above and on the structural and electronic properties of the intrinsic Si-layer material. Indeed, transistor response is defined by the threshold voltage and by the charge-carrier mobility. Variation in TFT layer thickness and in TFT material property along various areas of the backplane will lead to variations of the transistor properties. This will result in a non-uniformity of pixel luminance over the panel.
The limitations which have been described with respect to the μc-Si layer deposition up to now as from halogen precursor gas are substantially remedied by the manufacturing method according to the present invention and foreseen for industrial-scale production.
The dielectric precoat, which is applied to cover at least parts of the inner surface of the reactor, leads to a substantially ameliorated uniformity of the chemical environment as “seen” from the substrate and along its entire surface to be coated.
When we speak of providing a dielectric precoat on “at least a part of the inner surface of the reactor”, we do not want to exlude that some areas of such inner surface will not be covered by the precoat, which uncovered parts being of no or neglectable relevance for the electrochemical environment which the overall surface of the substrate to be coated sees during processing.
In one embodiment of the method according to the present invention the reactor is provided with a substrate carrier first electrode and, spaced therefrom, with a gas shower second electrode and the substrate is introduced in the addressed step c) on the first substrate carrier electrode.
Here the substrate is deposited onto the dielectric precoat applied to the carrier electrode. Customarily the surface of the precoat-covered first electrode is larger than the surface of the substrate deposited thereon. Considering the dielectric layer deposition in addressed step d), there results that the subsequent μc-Si layer deposition “sees” practically continuously a dielectric surface along the substrate and along the projecting precoated electrode surface as well as along wall areas of the reactor, neighboring the carrier electrode and the substrate edge. Thereby electric field uniformity too, especially along the edge of the substrate, e.g. of glass material, is greatly improved.
Due to the manufacturing method of the invention substrates of large surface result, the silicon layer thereon being provided as a μc-Si layer with improved uniformity of thickness and quality up to the edge of the substrate. This makes sure that electrical properties of devices formed on the basis of such silicon layer are of substantially improved local uniformity over the whole substrate area, and, due to the μc-Si deposition, are also of improved stability over time.
In one embodiment of the method according to the present invention at least one of depositing in step b) and of depositing in step d) a dielectric material is performed by PECVD processing.
Considering the fact that in step e) of the method according to the present invention, a μc-Si layer is deposited by PECVD, the overall processing is significantly simplified by making use of the same processing type at least for depositing the precoat according to step b) or to apply the dielectric layer according to step d) or to perform both addressed steps b) and d). Nevertheless, in some circumstances it might be preferred to deposit the precoat layer and/or the dielectric layer by means of e.g. reactive physical vapor deposition (PVD) using an RF plasma discharge as well.
When we talk throughout the present invention of applying or generating an RF plasma, this shall not exclude that besides of an RF supply there might be applied to the respective electrodes additional a DC bias or such RF plasma might be generated by pulsating RF or even pulsating DC of high enough repetition frequency.
Additionally, it has to be considered that for some reasons a microwave plasma might be used. Here microwave energy is coupled into the reactive space of the vacuum PECVD reactor. In this case the first electrode is rather used as a substrate carrier, the second electrode rather as a gas shower for homogeneously introducing the respective gas or gas mixture.
In a further embodiment depositing on the dielectric layer according to step e) a μc-Si layer comprises plasma activating a gas or gas mixture, which produces etching and layer growth-contributing radicals.
In one embodiment such gas is selected to be SiF4.
Still in a further embodiment of the method according to the invention in step e) a gas mixture comprising a silicon-containing gas, a halogen-containing gas and hydrogen is plasma-activated.
Still in a further embodiment of the invention step e) comprises activating a noble gas.
Still in a further embodiment of the invention at least the surface of the substrate to be coated is of glass, customarily today the entire substrate.
Still in another embodiment of the present invention at least one of step b) and of step d) comprises depositing of at least one of silicon oxide, silicon nitride, silicon oxynitride, fluorinated silicon oxide.
Still in a further embodiment of the method according to the invention step b) comprises depositing silicon nitride. Thereby, in one embodiment step b) comprises depositing the dielectric precoat with a thickness d for which there is valid:200 nm≦d≦500 nm.
Still in a further embodiment the addressed thickness d is selected to be200 nm≦d≦400 nm.
Thereby, it must be considered that the higher this thickness d is selected, the higher will be the risk of precoat material peeling off. Increasing the addressed thickness above the addressed 500 nm will further lead to a decrease in total throughput of the manufacturing equipment due, on one hand to long precoat times and, on the other hand to reduced RF power coupling through such precoat. Coating thicknesses below 200 nm on the other hand have proven to be inefficient to improve the targeted thickness and quality uniformity sufficiently, i.e. to be below a standard specification requiring thickness variation e.g. of less than 10% of an average thickness value considered along the overall substrate surface.
In a further embodiment of the method according to the invention step b) comprises depositing the dielectric precoat as a layer of amorphous material. Thereby, the stress within the precoat layer is significantly reduced. This leads to an improved adherence of such precoat layer to the vacuum reactor metallic inner surface.
In a further embodiment the material of the precoat deposited in step b) and of the dielectric layer deposited in step d) are equal at least considering their composition. Thereby, they need not be equal in structure. Thus, as was addressed the precoat may be deposited as amorphous material which is not done necessarily for the dielectric layer deposited in step d).
Although not always mandatory, in most cases a further embodiment of the present invention comprises plasma cleaning at least the addressed parts of the inner surface of the reactor which are afterwards coated with the dielectric precoat, thus performing such cleaning before performing step b).
In an embodiment the addressed cleaning is performed in plasma-activated SF6 and oxygen.
The method according to the present invention with all embodiments addressed is highly suited for manufacturing thin film transistor display substrates or liquid crystal display substrates or solar cell substrates or organic light-emitting display panels. Thereby, at such substrates the addressed μc-Si layer becomes the intrinsic Si-layer of semiconductor devices, which as perfectly clear to the skilled artisan, are realized following up step e) by additional layers deposited, preferably in the same reactor and without vacuum interruption. Therefore, it has to be emphasized that between the addressed step e) and the addressed step f) one or more than one additional treating steps may be performed before manufacturing of the flat substrate is terminated and before step f) is performed for a subsequent single substrate to be manufactured.
FIG. 1 schematically shows a simplified vacuum reactor 1 as may be used to operate the method according to the present invention. The vacuum reactor comprises a surrounding reactor wall 3 which is metallic and customarily made of an aluminum magnesium alloy. Within the vacuum reactor there operates a two-dimensionally extended gas shower electrode 5, which, as schematically shown in FIG. 1, is at least in part electrically isolated from the wall 3 of the vacuum reactor 1. The gas shower electrode 5 is on one hand electrically operationally connected to a power supply unit 7 and on the other hand to a gas supply as schematically shown by the arrow G. The respective gas or gas mixture is introduced to the gas shower electrode 5 and is inlet through a multitude of gas inlet openings g to the reaction space R of the vacuum reactor. Uniform gas distribution along the reaction space R is e.g. achieved by respective two-dimensional distribution of the gas inlet openings g via a distribution chamber 9 in the gas shower electrode 5. Opposite to the gas shower electrode 5 there is provided in the vacuum reactor 1 a substrate carrier electrode 11 which is either operated on the same potential as wall 3 of the vacuum reactor or which is operated at an electric potential different therefrom, which necessitates, in such case, electrical isolation of the substrate carrier electrode 11 from the wall 3 of the vacuum reactor 1. By means of the supply unit 7 electric power is applied to the two electrodes 5 and 11 as suited to deposit electrically non- or at least very low-conductive layers. Thus, the supply unit 7 provides for an electrical RF supply of the electrodes 5 and 11 or for an RF supply with DC bias or for pulsed RF supply or for a high-repetition frequency pulsed DC supply etc.
Additional members which are mandatory for a vacuum reactor as e.g. vacuum pumping port, input/output loadlock etc., are not shown in the schematical FIG. 1 as perfectly known to the skilled artisan.
FIG. 2 shows a flow-chart of the method according to the present invention. In a first method step there is generated an RF plasma discharge with one of the suited electrical supplies as schematically shown in the supply unit 7, within the reaction space R between the gas shower electrode 5 and the substrate carrier electrode 11. With the help of such RF plasma discharge and inletting a respective reactive gas into the reaction space R, there is deposited on at least a part of the inner surface of the reactor a dielectric precoat 13. Such precoat 13 is shown in FIG. 3 at those relevant parts of the reactor type of FIG. 1, where it is mandatory. Thereby, as obvious, the thickness of the addressed precoat 13 is largely exaggerated for clearness sake.
With an eye on the steps of generating an RF plasma discharge in the reaction space between the addressed electrodes and inletting a respective gas or gas mixture, thereby depositing the addressed dielectric precoat, this should not be understood as being done in the time sequence according to the sequence of wording and mentioning such steps here. E.g. it may be possible to first inlet the gas or gas mixture to the reaction space and then to establish the RF discharge or to establish the RF discharge and then to inlet the gas or gas mixture. Important is that deposition of the precoat starts when both conditions are met, namely RF discharge established and gas or gas mixture present in the reaction space R.
The dielectric precoat 13 must be applied along at least the peripheral area of the substrate carrier electrode 11 and along the surface areas of the wall 3, which laterally surround the edge of the substrate carrier electrode 11.
Especially when making use of a vacuum reactor type as shown in FIG. 1 the dielectric precoat is PECVD deposited, thereby of at least one of the following materials: silicon oxide, silicon nitride, silicon oxynitride, fluorinated silicon oxide. In today's embodiments silicon nitride is used. The thickness of the precoat as applied, d, is200 nm≦d≦500 nm
and thereby in a today's embodiment200 nm≦d≦400 nm.
PECVD processing is thereby controlled so as to lead in today's embodiment to an amorphous material structure of the addressed precoat 13.
Today, before every such precoating, there is performed RF plasma reactive cleaning of the metallic inner surface of wall 3 e.g. making use of a gas mixture comprising SF6—O2. Nevertheless, and as addressed later, such cleaning before each precoating step—which latter is mandatory before each single substrate treatment—may be omitted and replaced e.g. by a cleaning step after each third substrate or even just when it becomes necessary.
With an eye on the parts of the wall 3 which are, according to the present invention, to be covered by the dielectric precoat, the parts as shown at 13 in FIG. 1, are substantially those of the reactor type according to FIG. 1, which mandatorily have to be covered to fully exploit the effect according to the present invention. According to FIG. 2 and in a further step after precoating there is introduced one single large surface substrate with an extent of at least 2500 cm2 e.g. of glass into the reactor and, making use of the reactor type of FIG. 1, is deposited on the substrate carrier electrode 11. Thereby, in today's operated embodiments, the substrate 15 is smaller than the substrate carrier electrode 11, so that latter protrudes all around the substrate.
In FIG. 3 there is shown, schematically, an enlarged portion of the periphery area of the substrate carrier electrode 11 coated with the dielectric precoat 13 and with the lateral area of the precoated vacuum reactor wall 3. The substrate 15 is introduced and is deposited on the substrate carrier electrode 11. Then the substrate carrier electrode 11 the substrate 15 and thereby especially its upper surface 15o is coated with a dielectric layer 17. In today's embodiments the dielectric layer 17 as shown in FIG. 3 is made of at least one of a silicon oxide, silicon nitride, silicon oxynitride, fluorinated silicon oxide. The same material may be selected as has been deposited as precoat 13. Nevertheless, the material structure of precoat 13, which is in today's embodiments amorphous so as to reduce internal stress and thereby improving adhesion to the metallic surface of wall 3, the dielectric layer 17 deposited upon the substrate 15, again by PECVD when making use of the reactor type as shown in FIG. 1, needs not be the same but may be crystalline.
The substrate 15 is in today's manufacturing of glass.
Note that already when depositing the dielectric layer 17 upon the substrate 15 there is substantially no metallic surface of wall 3 which may be seen from the reaction space 3 adjacent to the substrate 15 to be coated. Therefore, no such metallic surface may influence the etching to deposition equilibrium when depositing the dielectric layer 17 upon the substrate 15. Thereby, already the dielectric layer 17 is deposited with a substantially uniform thickness and with a substantial uniform material characteristics all along the surface 15o of the substrate 15.
After having performed dielectric layer deposition on the substrate 15 upon the addressed dielectric layer 17 a silicon layer of μc-Si is PECVD deposited. This μc-Si layer is shown in FIG. 3 in dash lines at reference No. 19. It has to be emphasized that when, by PECVD the μc-Si layer 19 is deposited, the reaction space R does again not see any metallic surface of wall 3 which would change the etching to deposition equilibrium along the peripheral area of substrate 15. Thickness and structure of the μc-Si layer becomes uniform along the substrate surface 15.
For depositing the Si layer 19 as a μc-Si layer etching radicals are exploited. In order to achieve a fully crystallized material structure of layer 19, it is grown from a gas or gas mixture in today's embodiment from SiF4, which produces growth-contributing radicals (silicon-containing radicals) as well as etching radicals (fluorine-containing radicals). Although the growth mechanism is not fully understood it is known that the growth is mainly governed by the balance or equilibrium between etching radicals and deposition radicals (see P. Roca i Cabaroca et al.) Different radicals/surface interactions may occur when a metallic surface, as an aluminum magnesium surface of the reactor wall—including electrode surfaces—is exposed to the activated gas on one hand and, on the other hand, substrate material, as glass, is exposed to the same plasma-activated gas. As in a standard PECVD reactor e.g. of the type as shown in FIG. 1, the substrate is lying on the substrate carrier electrode 11 and with an eye on FIG. 3, such etch-to-deposition balance would be extremely perturbed near the edge of the substrate 15, if the precoat 13 was not present. The precoat first improves uniformity of the dielectric layer deposition and then improves, combined with such dielectric layer, uniformity of μc-Si layer deposition.
It has further to be noted that the precoat step, the deposition step for the dielectric layer on the substrate, the deposition step for the μc-Si layer as well as the deposition steps for subsequent layers before repeating all the addressed steps for a further single substrate, are advantageously performed in one and the same vacuum reactor.
In context of FIG. 1 we have shown a customarily used PECVD vacuum reactor, in fact a parallel electrode PECVD reactor. It goes without saying for the skilled artisan, that other and different known PECVD reactor types might be used for operating the present invention. Practically always, in PECVD layer deposition on a substrate as addressed there will be present at the edge of the substrate substrate material and metallic material of the recipient equipment, making precoating as was described necessary to improve especially μc-Si layer uniformity. As an example, instead of a parallel plate PECVD reactor of the type as shown in FIG. 1 it is possible to apply a PECVD reactor, whereat the gas or gas mixture is activated by a microwave plasma.
FIG. 4 shows the non-uniformity of a μc-Si layer near the edge of a glass substrate. On a glass substrate of 720 mm×650 mm a stack of 200 nm SiN and of 105 nm μc-Si was deposited in a PECVD reactor principally as shown in FIG. 1. There was used a reactor type KAI 1XL as commercially available from the applicant. The carrier electrode 11 was larger than the substrate by 1 cm on each of the sides. No precoating was performed. The large non-uniformity region at the edges of the substrate is obvious, where the thickness average decreases from a value of 105 nm to as low as 80 nm.